Thermodynamics – Objective Questions

Page 7 : Tricks, Shortcuts & Solution Methods

🔥 TRICK 1 – Internal Energy Shortcut

For ideal gas, internal energy depends only on:

• ✔ Temperature
• ✘ Pressure
• ✘ Volume

Shortcut: If temperature same → ΔU = 0 (no calculation needed)

🔥 TRICK 2 – Cyclic Process Rule

In any cyclic process:

• ΔU = 0
• Net heat = Net work

IIT Trap: Never calculate internal energy in a full cycle.

🔥 TRICK 3 – Adiabatic Test

If question says:

• No heat exchange
• Very fast process
• Perfect insulation

Direct conclusion: Adiabatic → Q = 0

🔥 TRICK 4 – Steeper Curve Rule (P–V)

Between isothermal & adiabatic:

• Adiabatic curve is always steeper

IIT Trap: Many students mark isothermal by mistake.

🔥 TRICK 5 – Sign Convention Master

• Expansion → Work done by gas (+)
• Compression → Work done on gas (–)
• Heat absorbed → +Q
• Heat released → –Q

Q61. A gas expands freely in vacuum. Which is true?

• A) Temperature increases
• B) Work done is zero
• C) Heat absorbed
• D) Pressure constant

Answer: B

Trick: No external pressure ⇒ W = 0

Q62. In isothermal expansion of ideal gas:

• A) ΔU ≠ 0
• B) Q = W
• C) W = 0
• D) Q = 0

Answer: B

Shortcut: ΔU = 0 ⇒ Q = W

Q63. Which process gives maximum work output?

• A) Isochoric
• B) Adiabatic
• C) Isothermal
• D) Free expansion

Answer: C

Logic: Isothermal allows heat input during expansion.

Q64. Efficiency of Carnot engine depends on:

• A) Nature of gas
• B) Amount of gas
• C) Temperatures only
• D) Pressure

Answer: C

Formula: η = 1 − (T_cold/T_hot)

Q65. Which process has zero work?

• A) Isothermal
• B) Adiabatic
• C) Isochoric
• D) Cyclic

Answer: C

Reason: Volume constant ⇒ no work.

Page 7 Completed ✔
These tricks alone can save 10–15 minutes in JEE Advanced

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🔥 Thermodynamics – Objective Final Revision

Thermodynamics – Set 1
Set 2
Set 3
Set 4
Set 5
Set 6
Set 7
Set 8
Set 9
Set 10
Set 11
Set 12
Set 13
Set 14
Set 15
Set 16
Set 17
Set 18
Set 19
Set 20
Set 21
Set 22
Set 23
Set 24
Set 25
Set 26
Set 27
Set 28
Set 29
Set 30
Set 31
Set 32
Set 33
Set 34
Set 35
Set 36
Set 37
Set 38
Set 39
Set 40
Set 41
Set 42
Set 43
Set 44
Set 45
Set 46
Set 47
Set 48
Set 49
Set 50
Set 51
Set 52
Set 53
Set 54
Set 55
Set 56
Set 57
Set 59

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